1. Field of the Invention
The present invention relates to power supplies and methods of controlling the same, and more particularly to a power supply that supplies electric power to a load by parallel operation of multiple power supply units and to a method of controlling the same.
2. Description of the Related Art
In recent years, it has been common for power supplies having a DC-DC converter (hereinafter simply abbreviated as “DDC”) applied thereto to have multiple small on-board DDCs on each printed circuit board because of the necessity of distributed power supply. The output current of each of these on-board DDCs is 40 to 70 A, and electric power is supplied to a high-end UNIX (registered trademark) server, the CPU/LSI of a mainframe, etc., by parallel redundant operation of two through eight on-board DDCs.
In such power supplies, it is a common practice to connect multiple power supply units (Power Supply 1, Power Supply 2, . . . ) in parallel, that is, to connect the output parts (Output Part 1, Output Part 2, . . . ) of the power supplies in parallel, as shown in FIG. 1, in order to output a large current.
Recent loads are of low voltage and large current. Therefore, it is often the case that a so-called “simple output connecting method” optimum for large current is applied to the output part of each of the power supply units connected in parallel as described above in place of a so-called “diode OR-ing method.”
In the case of the “simple output connecting method,” it is impossible to operate a power supply in a redundant manner. Unlike the “diode OR-ing method,” however, it is possible to eliminate loss at a parallel connection point. Accordingly, it is a method optimal for a power supply system for large current.
On the other hand, this simple output connecting method cannot prevent a backflow of current in the case of a short-circuit failure (point X in the drawing) inside one of the power supply units as shown in FIG. 2. This makes it impossible to operate a power supply in a redundant manner. That is, in such a case, a current exceeding the supply capacity of a normal power supply unit flows from the normal power supply unit to the failed power supply unit. As a result, an overcurrent protection circuit operates inside the normal power supply unit, so that eventually, the normal power supply unit also stops its outputting.
That is, according to the “simple output connecting method,” with the stoppage of a failed power supply unit, other normal power supply units in parallel operation also stop outputting. This causes the problem of the inability to distinguish the actually failed power supply unit.
In the case of the above-described “diode OR-ing method,” for example, diodes D1 and D2 are provided in the output parts (Output Part 1, Output Part 2 . . . ) of power supply units as shown in FIG. 3. Therefore, if there is a short-circuit failure inside one of the power supply units, a backflow of current can be prevented by the diode D1 or D2. As a result, a normal power supply unit can continue to supply output voltage to a load without stopping outputting. Further, it is also possible to distinguish a failed power supply unit according to this method.
According to this “diode OR-ing method,” however, because of the forward voltage VF of the OR-ing diode D1 or D2, a loss P=Io1×VF at the time of large current output increases. This may result in an increase in the volume or price of the power supply unit because of the need for a heat sink for cooling. Accordingly, this is not necessarily an appropriate method if there is no need for redundant operation.
[Patent Document 1] Japanese Laid-Open Patent Application No. 2003-169471